1. Field of the Invention
The present invention relates to equipment and the relative process for the preparation of electric conducting composite samples starting from powders, (this term also comprising lyophilized organic compounds), in the form of matrices or as a carrier by impregnation, adsorption or dispersion of liquids or by impregnation or adsorption of gases, which may or may not be insulating and of an organic or inorganic origin, to be subjected to instrumental chemical analysis with techniques for the direct analysis of solid conductors.
2. Description of Prior Art
Various advanced instrumental techniques and/of of consolidated use, such as, for example, AAS, GFAAS, ICP-OES, ICP-MS, LC-ICP-MS, voltammetry, etc. can be used for the chemical characterization of organic and inorganic materials and in particular for the quantitative determination of element traces: in all cases there is dissolution of the sample and dilution in water.
Some alternative anlaytical techniques, capable of reaching detecting limits (LoD) for many elements of less than 1 ppb, such as SIMS, SSMS, GDMS, allow direct analysis of the sample without preliminary chemical attack and subsequent dilution.
An application limit of the latter pair of techniques is due to the necessity for the material under examination to be a (semi)conductor. In the contrary case, the matrix must be manipulated so as to obtain a sample for analysis which is conducting; for this purpose it is possible:
to mix it with ligand metals (Ti, Cu, Ga, Ag and Ta are those most commonly used) or with graphite, PA1 to couple it to a secondary cathode, exploiting the partial redeposition effects of the vaporized metal coming from the cathode on the insulating sample, PA1 to subject it to infiltration, according to the preparative procedure described in "An innovative sample preparation procedure for trace and ultratrace analysis on non-conducting powders by direct current glow discharge mass spectrometry", (Battagliarin M., Sentimenti E., Scattolin R.--Spectrochimica Acta, Vol. 50B, Nr. 1, pages 13-25, 1995). PA1 introduction of the powder to be analyzed into a tube in PTFE of the equipment; PA1 insertion of a rod of low-melting metal ligand into the tube; PA1 vapor and gas evacuation and subsequent heating to melt the ligand; PA1 pressurization for the infiltration of the powder; PA1 cooling and extraction of the sample for analysis. PA1 it is essential to apply very high pressures, on an average not less than 10-15 tonn/cm.sup.2, with considerable stress of the moulds and relative contamination of the sample on the part of the material forming the mould itself; PA1 it is necessary to eliminate these surface contaminations by means of a vigorous chemical attack and a prolonged sputter etching; PA1 it is necessary to carefully clean the mould to minimize any possible residual effects; PA1 the ligands which are most suitable for the purpose and commercially available at reasonable prices, except for gallium and graphite, do not easily reach the desired degree of purity; PA1 a significant dilution of the sample is introduced with a decrease in the LoD; PA1 it is particularly difficult to obtain a satisfactory homogenization of the mixture resulting in a deterioration in the repeatability of the measurements; PA1 it is necessary to find the correct weight ratio ligand/matrix to have good mechanical resistance of the sample; PA1 traces of water adsorbed on the powder granules can significantly lower the sputtering yield and slow down the vaporization rate. PA1 it is essential to pellet the material to be analyzed by pressing with suitable moulds; PA1 residual effects are easily generated in the material in the mould, which must be eliminated with suitable chemical attacks; PA1 with this geometry the sputtering yield and detecting limits are lower than those obtained from rods; PA1 the selection of the material forming the cathode, the dimensions of the central hole (focal spot) of the cathode/mask, the presputtering and discharge conditions are extremely critical; consequently the set-up of an analysis protocol is rather complex; PA1 it is extremely important to have a mask made of material with a very high purity; PA1 optimum operating parameters vary considerably from matrix to matrix. PA1 equipment is used which already exists (designed for the preparation of composite materials with a metallic matrix [MMCs], for mechanical components by infiltration with molten aluminum and its alloys) which is greatly overdimensioned and cannot be easily installed in a sample preparation area of group 100 (for example, glove box); PA1 the design does not allow efficient control of the process development and it is therefore necessary to excessively prolong the residence times of the charge in the chamber to obtain complete melting of the ligand before applying the necessary pressure for infiltration; PA1 the material used for the mould (PTFE) is easily deformed and can tolerate only a few operating cycles; the main components must be frequently substituted; PA1 the seal (O-ring in Viton) does not ensure perfect insulation between the pressurized environment and the base of the charge; PA1 the molten ligand cannot be adequately confined once compressed; this results in poor reproducibility of the end-dimensions of the sample for analysis and a consumption of ligand of more than 25-30% with respect to the useful quantity; PA1 the molten ligand may come into contact with the seal; PA1 the degree of vacuum which can be reached is relatively low (1-10 mbars), as is also the rate at which the gases adsorbed on the matrix are removed. PA1 a pressurization/infiltration chamber having at least one opening to connect it by means of appropriate pipes with a vacuum line and a pressure line and devices for sealing both under vacuum and under pressure; PA1 a sample-holder, inside said chamber, made of a material with a high thermal conductivity with one or more seats, preferably vertically positioned, in each of which a mould is housed consisting of: PA1 means for creating a vacuum in the chamber; PA1 means for pressurizing the chamber; PA1 means for the thermal treatment of the sample-holder. PA1 charging a tube with the powder to be analyzed, after inserting the plug and spacer, this operation being effected either before or after the mould has been placed inside the sample-holder; PA1 introduction of a solid ligand into the tube in the area above the powder; PA1 closing the infiltration chamber after positioning a sealing device; PA1 evacuation from the chamber of the vapor and gases adsorbed on the powder grains bringing the chamber under vacuum, preferably to a pressure of less than 5.10.sup.-2 mbars, more preferably equal to or less than 3.10.sup.-2 mbars.; PA1 heating the supporting base of the chamber and consequently both the sampleholder and the mould in order to melt the ligand; PA1 pressurization of the chamber up to a pressure of at least 30 bars, preferably between 40 and 50 bars; PA1 extraction of the mould from the sample-holder, followed or preceded by cooling until the ligand has solidified, removal of the tube plug and extraction from the mould of the composite sample with consequent removal of the spacer. PA1 reduction or oxidation reactions or any reaction which can produce compounds with more thermal stability; PA1 lyophilization; PA1 preconcentration.
The equipment described in the article cited above essentially consists of a high pressure container which houses a mould containing the powder and metal to be melted and compressed.
The process using this equipment essentially comprises the following steps:
As an alternative to the above methods, sources can be used with discharge in alternating radio-frequency current (rf GDMS); the results so far obtained however, even if promising, are only partial and cannot be transferred to a commercial level.
For minimum quantities of material however, the infiltration system as a supporting means of the material of the sample is extremely useful if not indispensable.
Both the introduction of a conducting ligand and resort to a secondary cathode have a series of drawbacks which limit the instrumental performances.
The main disadvantages relating to each preparative method are summarized below:
For coupling with a secondary cathode:
For infiltration in PTFE mould: